1. Field of Invention
The invention relates to an endfire antenna and, in particular, to a low-sidelobe dual-band and broadband flat endfire antenna operating a dual-band environment.
2. Related Art
The prosperous development in wireless transmission has brought us various kinds of multi-frequency transmission products and technologies. Many new products are built in with the function of wireless transmissions. The antenna is an important element for a wireless transmission system to emit and receive electromagnetic (EM) wave energy. Without the antenna, the wireless transmission system will not be able to emit and receive data. Therefore, the antenna is indispensable for wireless transmissions. Besides fitting to the product shape and enhancing transmissions, using an appropriate antenna can further reduce the product cost. The designs and materials of antennas vary in different products. As different countries use different bands, one has to take that into account to design the antennas. Commonly used standards of the bandwidths include IEEE 802.11 and the hottest Bluetooth communications (802.15.1). The Bluetooth technology works in the 2.4 GHz band. The 802.11 standard is further divided into 802.11a and 802.11b, defined for the 5 GHz band and the 2.4 GHz band, respectively.
Antennas can be divided into uni-directional antennas (or traveling wave antennas) and omni-directional antennas (or resonant wave antennas). Moreover, uni-directional antennas have several kinds of variations, such as the tape antennas or endfire antennas.
A well-known uni-directional antenna is shown in FIG. 1. A tape antenna 10 is disposed on a dielectric substrate 15. A grounded plane 16 and a tape plane 17 are formed on opposite surfaces of a plate. The grounded plane 16 and the tape plane 17 are comprised of several thin metal plates of millimeter (mm) thick. The side length of the tape plane 17 is λ/2 (where λ is the wavelength of radio waves). A hole (not shown) is formed in the middle of the dielectric substrate 15. The core conductor 20 of a coaxial cable 19 goes through the hole and connects to the tape plane 17. Another hole 23 is formed on a support substrate 22. The coaxial cable 19 is installed by penetrating trough the hole 23. Moreover, the outer conductor of the coaxial cable 19 is connected to the grounded plane 16. The support substrate 22 is an insulator. A dielectric slab 27 is fixed on the support substrate 22 by a separator 26 disposed at corners of the substrate 22.
The above-mentioned uni-directional antenna can only operate in a single frequency band and is thus impractical for modem uses. Besides, the antenna involves high precision. Conventional manufacturing methods tend to be more complicated. Any errors in sizes or assembly alignment may change the operating frequency band of the antenna. All such effects will eventually increase the assembly cost of the antennas.
Therefore, it is an important subject in the field to provide a dual-band uni-directional antenna that has an increased operating bandwidth and a lower assembly cost.